The present invention relates to ceramic media, such as used in mass transfer applications and particularly to applications involving exposure to operating conditions containing halogens or halogen halides. Mass transfer, in the context of this Application, can mean separation of a component from a mixture of liquids or gases or the extraction of heat from a fluid flow. The ceramic media can be in the form of packing elements, such as those shapes commonly used in mass transfer applications, or other random or structured packing element shapes. The ceramic media could alternatively be in the form of bed support media. Without prejudice to the generality of their application in such fields, the ceramic media of the invention are particularly useful in the context of regenerative thermal oxidizers (xe2x80x9cRTO""sxe2x80x9d) in which a gas flow containing halogens, (typically chlorine, but with some bromine and/or fluorine components possible), and the correlative hydrogen halides.
RTO units are becoming more important as the drive to clean up effluent gases and to-conserve energy becomes more urgent. In an RTO unit an effluent gas containing combustible or pyrolyzable materials is cycled through a first chamber containing packing elements that has previously been heated and thereafter enters a combustion chamber where the combustible or pyrolyzable materials are burned. The effluent gases then pass through a second chamber containing packing elements. These absorb at least some of the heat from the gases before the effluent is discharged to the atmosphere of for further processing. When the elements have reached an elevated temperature such that heat transfer no longer occurs efficiently, the flow direction is reversed and the second chamber becomes the first chamber and vice versa.
The packing elements in the chambers of the RTO can be in the form of monoliths with a plurality of through passages that are stacked within the chamber to provide a plurality of rectilinear parallel passages through which the gas can flow on its way through the chamber. Alternatively and often preferably the elements are relatively small individually and are dumped in random fashion within the chamber so as to provide a large number of non-rectilinear routes through the chamber for the gas. The individual elements can have a wide range of shapes such as hollow cylinders, with and without internal septa or other internal structures, cylinders with triangular or xe2x80x9cbow-tiexe2x80x9d crosssections, and porous pellets.
Gas flows that are particularly suitable for treatment using RTO""s may be generated for example when gas flows containing combustible materials that include halohydrocarbons are burned in an RTO unit as part of an effluent purification process. In such applications it is necessary that the elements used in the RTO are capable of absorbing heat rapidly and are stable under thermal cycling conditions as would be expected, but also that they are resistant to attack by the halogen-containing components of the effluent. This is important since replacement of the packing elements usually requires a shutdown of the RTO while the elements cool, are then extracted and replaced. Obviously the fewer times this has to occur, the better and more economically the unit operates. Furthermore, in the cases in which the ceramic elements in the RTO""s are attacked by the halogen-containing components of the effluent, but not so severely as to degrade the media to the point of necessitating a change-out, there are still problems that result directly from the ceramic-halogen reactions. The reaction of the halogens, such as Cl2 and other chlorine-containing gases with the elements inherently present in ceramic media, especially Na, K, and possibly Li, Ti and Fe, results in the formation of a precipitate, particularly NaCl and KCl, downstream from the thermal oxidizer. In the cases with downstream waste-heat boilers, this deposition of the precipitate causes a buildup and eventually fouling of the boiler, which causes a shut-down of the whole process for a clean-out.
Typical packing elements for RTO applications are made from clay/feldspathic material mixtures because these have good stability to thermal cycling while having a good capacity to absorb heat. They are, however, seriously attacked by atmospheres containing halogens or halogen acids. The present invention provides packing elements that are relatively stable to attack under such conditions and which therefore provide a significant advantage for treatment of hot halogen or hydrogen halide-containg effluents from catalytic or other processes for making or treating halogen-containing organic compounds.
The present invention provides a ceramic packing element having an alkali metal content that is not greater than 0.25% by weight, formed from a fired mixture comprising 10 to 98%, and preferably from 35 to 65%, by weight of a clay having an alumina content of at least 36% by weight; from 2 to 90%, and preferably from 35 to 80%, by weight of a talc containing at least 95% by weight of magnesium silicate as determined by X-ray diffraction analysis; and from 0 to 10%, preferably from 3 to 7%, of a dolomitic limestone containing at least 60 to 90% by weight of calcium carbonate and at least 10% and preferably 40 to 10%, by weight of magnesium carbonate and less than 10% of non-carbonate impurities.
The clay component is one that has an alumina content as received of at least 36%, which after calcination would be at least 42% by weight largely as result of the loss of free and bound water. The other major component, (at least 50% by weight), is silica but minor amounts of the oxides of calcium, magnesium, sodium, potassium, iron and titanium can also be present, usually in amounts of less than 1% each and, with respect to alkali metals, an amount that is rot greater in total than would lead to ceramic media with 0.25% or more of alkali metal oxides. The alkali and alkaline earth metal oxides are preferably present in amounts less than 0.2% by weight. A typical clay component suitable for use in this invention is a ball clay.
Before firing to produce the ceramic, the clay is preferably processed to a fine powder with at least 95% of the powder weight having particle sizes less than 10 micrometers and more preferably with at least 50% by weight having particle sizes less than 1 micron. The methylene blue index, (xe2x80x9cMBIxe2x80x9d), of the preferred clay, as measured according to the procedure in ASTM C-837, is at least 7.5 meq./100 gm, indicating reasonably good forming and shaping capability as compared to other clays containing at least 42% aluminum oxide after calcination.
Chemical analysis of preferred talcs for use in the ceramic elements according to the invention shows at least 60% by weight of silica, preferably from 60 to 66% by weight, and at least 30%, preferably from 30-33%, by weight of magnesium oxide, measured by X-ray fluorescence. Alkali metal oxides preferably account for less than 0.1% by weight of the talc. The loss-on-ignition of the talc, largely as a result of elimination of free or chemically bound water, is typically less than about 9%, such as from 1 to 9%, by weight with the preferred talcs. The talcs preferred for use in the ceramics according to the invention have a particles with sizes such that at least 95% by weight are 200 mesh, (74 micrometers), or finer.
The dolomitic limestone, which term is intended to convey a mixed carbonate of magnesium and calcium in a weight ratio of these carbonates of from 1:5 to 1:7, may be present in amounts of up to 10% by weight, for example from 2 to 8% by weight. It is preferred that the iron oxide content of the limestone be less than 1% by weight and that of the alkali metal oxides be less than 0.5% by weight. It is preferred that the limestone be processed prior to formation of the ceramic to a powder with particles sizes in which at least 95% by weight, and more preferably at least 99% by weight, are 325 mesh, (44 micrometers), or smaller.
To make the ceramic packing elements according to the invention the components are measured out by weight and thoroughly mixed before water is added in an amount that is sufficient to enable the mixture to be shaped into the desired form and to retain that form during firing. Generally this implies that the amount of water added should be from 12 to 30 ml for every 100 gm of the dry mixture of the components. It is also possible, though generally unnecessary, to add extrusion aids or other flow agents to make the subsequent shaping process easier and to confer some added green strength to minimize slumping during firing.
The shapeable mixture can then be molded, or preferably extruded to form the desired shape before the shape is fired in a kiln to a maximum temperature of from 1100xc2x0 C. to 1400xc2x0 C. The temperature in the kiln usually increases at a rate of between 50 to 90xc2x0 C./hr. and the dwell time at the calcining temperature is usually from 1 to 4 hrs before the kiln is allowed to cool to ambient temperatures.